Everyone seems to think that the ubiquitous L-pad is the only solution to the age-old problem of matching a 107dB horn to a 95dB woofer. By adding a 12dB resistive attenuator at the horn's driver it will play at a level matched to the bass. The speakerformers offer a magnetic solution to this situation and I will present a couple of the technical benefits below. For a general overview on what an autoformer is please check out the Autoformer FAQ.
Simply put, damping factor is the ratio of speaker impedance to amplifier output impedance. Autoformers do one simple thing and that is turn any excess voltage into the ability to deliver current. Resistive networks simply take that excess voltage and turn it into heat. Along they way they also add appreciable series impedance to the amplifier netting you a single digit damping factor. Lets take the case of the 12dB attenuation required in the situation above and consider what happens to damping factor with an autoformer vs. an L-Pad into an 8Ω driver.
For this situation lets assume a tube amplifier with a 0.5Ω output impedance which would give a damping factor of 16 if connected directly to the driver. The equivalent circuits with their respective attenuations are pictured below. It is important to note that the typical 12dB L-pad calculation actually nets 12.5dB of attenuation due to the 0.5Ω driving impedance.
What becomes interesting is the actual drive impedance that the speaker sees in both cases. The addition of the L-pad takes the amplifiers nominal output impedance of 0.5Ω and makes it nearly 2Ω which takes the damping factor of 16 and makes it 4. The autoformer on the other hand takes that 0.5Ω and reduces by a factor of 15 to 0.038Ω for a damping factor of over 200. I am not going to devolve into the world of specsmanship but the difference is substantial.
Crossing Over / Rshunt
One of the main considerations of using an autoformer for driver matching is that in addition to the voltage going down, the reflected impedance goes up. This needs to be accounted for which requires the introduction of a shunt resistor (Rshunt) across the autoformer that is installed by the end user in the spot provided on the board. It is important to remember that this resistor will dissipate power so select an appropriate wattage part and monitor it for heat when first installed.
The value of this resistor is dependent on attenuation level and the calculator below will give you the proper value of Rshunt to install on you speakerformers. An 8Ω driver with 12dB of attenuation will appear to the crossover as 127Ω. This means that the value of Rshunt will need to be 8.5Ω to keep the crossover points the same. If you move the attenuation slider a dB or two in either direction you will see that the Rshunt value changes slightly but this only becomes an issue at low levels (less than 6dB) of attenuation.
A unique benefit of the shunt resistor is you are no longer tied to the speaker's nominal impedance. In the case we have been discussing lets assume a crossover point of 2Khz. The plot below shows what happens when Rshunt is set for both an 8Ω and 16Ω impedance. Simply put the 16Ω value shifts the crossover frequency down an octave. If the crossover point is to remain the same, moving to 16Ω would require a crossover cap of 1/2 the value which opens the door for boutique caps at a more affordable price.
The value of Rshunt can also be used to tweak the crossover frequency. Below the values are swept to show some possible inter-octave values.
The speakerformers use a 10 position rotary switch and a trim toggle to net 20 possible settings. The knob gives the indication of the level. The trim will give the knob value if set to 0 or subtract what the toggle indicates. In the above picture the Right unit is set to -9.5dB. The following 4 options are available.
-3dB to -12.5dB step size 0.5dB
-6dB to -15.5dB step size 0.5dB
-9dB to-18.5dB step size 0.5dB
-1dB to -20dB step size 1dB
$350 a pair.
Low Attenuation Version
In the event that only a few dB of attenuation is required the selection of the shunt resistor needs to be changed for each step to keep the crossover frequency consistent. These units allow for 1dB to 6dB of attenuation in 1dB steps and the shunt resistor is adjusted for each step to keep the impedacne contant at either 8Ω or 16Ω. Custom 6 step ranges and impedances are possible, please email me to discuss.